In recent years, as peripheral equipment for personal computers—such as HDDs (hard disk drives)—AV equipment, household electric equipment, and the like have become lightweight, compact, and of higher performance, demand for sintered rare-earth magnets represented by Nd-based (neodymium-based) magnets has sharply increased. Typical alloys for such magnets are the Nd—Fe—B-type alloys, which additionally contain iron and boron and are typified by a composition Nd2Fe14B.
In many of these rare-earth magnets, in order to improve magnetic properties, to enhance economical efficiency through effective use of rare-earth elements, which are limited resources, and to enhance use-related properties (such as heat resistance and corrosion resistance), Dy (dysprosium), Pr (praseodymium), or similar rare-earth elements are incorporated so as to substitute for some portion of Nd, and Co, Al, Cu, or like elements are incorporated so as to substitute for some portion of Fe.
When there is no particular reason to limit a rare-earth element contained in rare-earth magnets including those mentioned above to Nd, the rare-earth magnets are collectively referred to as R-T-B-type magnets (R: rare-earth element; T: transition metal element).
Generally, all industrially produced R-T-B-type magnets contain R in an amount slightly exceeding the stoichiometric amount for the composition R2T14B. Thus, in a magnet alloy ingot, a phase which contains a rare-earth element(s), represented by R, at high concentration (hereinafter called the R-rich phase) is generated.
The R-rich phase is known to play the following important roles in R-T-B-type magnets.    (1) Since melting point of the R-rich phase is low, the phase becomes a liquid phase during sintering in a magnet production step, thereby contributing to achievement of high density of the resultant magnet and thus to improvement in remanence.    (2) The R-rich phase functions to smoothen grain boundaries, thereby reducing the number of nucleation sites in a reversed magnetic domain. Moreover, being nonmagnetic, the R-rich phase magnetically insulates the main phase, thereby enhancing the coercivity.    (3) Since the R-rich phase expands through absorption of hydrogen, this feature is utilized for decrepitating an ingot into pieces. Specifically, the R-rich phase is caused to absorb hydrogen so as to expand. As a result, cracks are generated within an alloy ingot, thereby decrepitating the ingot into pieces. The R-rich phase serves as a starting point of so-called hydrogen decrepitation.
In recent years, R-T-B-type magnets of improved magnetic characteristics, particularly R-T-B-type magnets of enhanced maximum magnetic energy product (BHmax), have been developed. In order to obtain such a high-performance magnet, the percentage of the R2T14B phase (hereinafter called the T1 phase), which produces magnetism, must be increased, and the R-rich phase must be reduced. In order to fulfill these needs, the total rare-earth element content (hereinafter called the TRE content) must be reduced so as to attain a near stoichiometric composition.
In such a case, the following problems that affect magnetic properties of the produced magnets are involved in alloy production steps and magnet production steps.
First, in melting and casting of an alloy; for example, a ternary alloy of Nd—Fe—B, the T1 phase forms through peritectic reaction between a primary γFe phase and a liquid phase. Thus, as the TRE content (the total R content) decreases, an αFe phase, which is a transformed form of γFe, tends to form. The αFe phase appears in the form of dendrites and extends three-dimensionally within the alloy, thereby significantly deteriorating crushability of the alloy in the magnet production step.
Second, when the TRE content is decreased, the percentage of the existing R-rich phase decreases. Thus, the aforementioned effects exerted by the R-rich phase; i.e., achievement of high density of the resultant magnet and enhancement in coercivity to a magnet, cannot be expected.
In order to solve the above problems, a strip casting process (SC process) has been developed (see, for example, Japanese Patent Application Laid-Open (kokai) Nos. 5-222488 and 5-295490). According to the SC process, a molten alloy is poured onto a water-cooled rotating roller of copper through a tundish and solidifies upon contact with the roll, so as to continuously produce a strip-like ingot. Subsequently, the strip-like ingot is crushed coarsely, and ultimately into flakes.
When an R-T-B-type rare-earth magnet alloy is cast by the SC process, very thin flakes, each having a thickness of about 0.2 mm to 0.4 mm, can be obtained, and therefore, cooling for solidification can be high. Thus, the molten metal can be cooled below a co-existence region of a liquid phase and γFe. That is, the T1 phase forms directly without formation of γFe. For example, a ternary alloy of Nd—Fe—B can be cast without formation of dendritic αFe while the Nd content ranges down to about 12.7 at. % (28.5% by mass), at which Nd content a high-performance magnet of 400 kJ/m3 or higher can be produced. (Y. Hirose, H. Hasegawa, S. Sasaki and M. Sagawa, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Volume 1, pages 77-86, 30 Aug.-3 Sep. 1998, Dresden, Germany).
Because of high rate of solidification, an alloy cast by the SC process has a relatively small crystal grain size of 20 μm to 30 μm as measured along the short axis. FIG. 7 schematically shows a cross-sectional structure of an R-T-B rare-earth alloy cast by the SC process and having an R content of 11.8 at. % (26.5% by mass) or more. In FIG. 7, the bottom surface (called the mold contact surface) is the surface of an ingot in contact with a mold, and the top surface (called the free surface) is opposite the mold contact surface.
Excess R over the stoichiometric amount in the composition R2T14B is diffused out from the solidification interface during solidification, thereby generating lamellar R-rich phases 30 arranged at intervals of 3 μm to 10 μm. The R-rich phases 30 form on the grain boundaries 28 of and within a crystal grain 29. As compared with a conventional alloy cast by means of a book-mold, the R-rich phases 30 are distributed finely and uniformly. Thus, crushability during hydrogen decrepitation is significantly improved, such that pulverized particles attain a size which is a fraction of the crystal grain size. That is, it is possible to obtain a powder constituted solely by single-crystal particles. A region denoted by reference numeral 32 is the T1 phase.
A powder consisting of single-crystal particles facilitates, in a later step of compaction in a magnetic field, formation of a compact which is oriented in the direction of the C-axis, which serves as an easy-magnetization axis.
However, mere mechanical pulverization disintegration without involvement of hydrogen decrepitation causes cracking to propagate through grains (i.e., penetrating grains) in the form of cleavage fracture without utilization of the R-rich phases generated on grain boundaries and within grains. As a result, among pulverized particles, an increased number of particles come to have crystal grain boundaries 28, or in other words, are not single crystal particles. Accordingly, the degree of alignment drops at the time of compaction in a magnetic field, causing an impairment in magnetization and a decrease in magnetic energy product after sintering.
The present inventors devised another rapid solidification process and an apparatus therefor (Japanese Patent Application Laid-Open (kokai) Nos. 08-13078 and 08-332557). Specifically, a molten material is introduced into a rotating mold via a box-like tundish, which is disposed in a reciprocative manner inside the mold and has a plurality of nozzles, whereby the molten material is deposited and solidifies on the inner surface of the rotating mold (CC (Centrifugal Casting) process).
In the CC process, a molten material is continuously poured onto an ingot which has already been deposited and solidified. The additionally cast molten material solidifies while the mold makes one rotation; thus, the rate of solidification can be increased. The newly poured molten material and the surface of the existing solidified ingot fuse together, whereby crystals grow epitaxially. Thus, the CC process can produce an alloy whose crystal grain size is several times longer than that of an alloy produced by the SC process.
However, in the production of an alloy of low R content, in contrast to the SC process, the CC process unavoidably involves formation of dendritic αFe due to low cooling rate in a high-temperature zone. For example, in production of a ternary alloy of Nd—Fe—B, formation of dendritic αFe is observed at an Nd content of about 14.4 at. % (31.5% by mass) or less, which is not observed in the SC process.
When the deposition rate of a molten material is decreased in order to increase the solidification-cooling rate in the CC process, the temperature of the solidified ingot drops, thereby increasing the temperature-dropping rate of the deposit layer of the additional molten material, leading to an increase in the solidification-cooling rate. However, decreasing the deposition rate in the CC process involves the following problems.
(1) Since the deposition rate is a value obtained by dividing the amount of supply (volume of supply) of a molten material per unit time by an effective inner surface area of a mold, the effective area of a mold may be increased. Specifically, a mold of large inside diameter or long length relative to the amount of the material to be cast may be used. However, this causes an increase in equipment scale, requiring a larger chamber. Also, the consumption of inert gas increases. Thus, economical efficiency becomes low.
(2) In order to decrease the deposition rate through decrease in the amount of supply of a molten material, the head of the molten material contained in a tundish must be lowered. In this case, the supply of the molten material becomes nonuniform, causing difficulty in obtaining an ingot of uniform thickness in the longitudinal direction of the mold. Accordingly, the deposition rate of the molten material varies in the longitudinal direction, resulting in nonuniform microstructure of ingot.
(3) When the amount of supply of a molten material is decreased, the temperature of the molten material contained in a tundish drops significantly, causing difficulty in performing stable casting.
(4) When the deposition rate is decreased, a rough surface of ingots tends to form, thus reducing commercial value.
The foregoing discussion is directed to R-T-B-type rare-earth magnet alloys. In recent years, demand for hydrogen storage mischmetal-nickel alloys has increased for use as materials for negative electrodes of nickel-hydride batteries, which are a type of secondary batteries. But regrettably, the hydrogen storage alloys also involve similar problems.
An intermetallic compound which serves as an important component of a hydrogen storage mischmetal-nickel alloy is a compound assuming an M1T5 phase, which contains a mischmetal M, which is a mixture of rare-earth elements, such as Ce, La, Nd, and Pr; and a transition metal T, which includes Ni as a fundamental element, at the ratio 1:5.
The transition metal T includes Ni as a main element as well as additional elements, such as Co, Al, Mn, and Cu, in order to adjust the equilibrium pressure associated with absorption and desorption of hydrogen and to improve catalytic characteristics in application to negative electrodes and various characteristics in application to batteries, such as charge-discharge cycle characteristics.
The M1T5 phase does not involve the problem of dendritic αFe formation, but, in casting through use of an ordinary book mold, involves the problem that Mn of added elements segregates, causing impairment in charge-discharge cycle characteristics. Thus, as in the case of magnet alloys, a rapid quench method and the SC process are proposed for production of hydrogen storage mischmetal-nickel alloy (Japanese Patent Application Laid-Open (kokai) No. 05-320792).
However, the proposed rapid cooling method involves the problem that residual strain tends to be locked in an ingot, causing impairment in hydrogen storage characteristics.
The present inventors also devised a method for producing hydrogen storage mischmetal-nickel alloys (Japanese Patent Application Laid-Open (kokai) No. 09-180716), making use of the CC process. However, the CC process unavoidably involves segregation of Mn due to slow solidification-cooling rate. A difficulty arises in uniformly melting any of metals of high melting point, such as Ti, Mo, Nb, V, W, Ta, and Cr, and alloys and intermetallic compounds which contain the metal(s).